Allows non-integral BFC delays (days)

This PR allows BasicFeedbackCondition feedback availability, credit window, and unavailability delays to be non-integral days (although they must still be greater than 0.0).

Existing BFCs should be unaffected, and no migration is required.  Upon update (even without changes), the delay text for existing BFCs will start reflecting the type change ("3 days" will become "3.0 days"). @kjdav

The code now uses explicit constructor calls (when available) to parse `store_typed_accessor` types.  These raise `ArgumentError` when the input string is not valid, and are less error-prone than the current method of doing comparisons using using `#to_<type>` and `#to_s`. @jpslav